The present invention relates to a fluid rotating apparatus to be used to discharge gas from a chamber such as a vacuum chamber of a semiconductor-manufacturing device.
A vacuum pump is used to provide a vacuum environment in a CVD apparatus, a dry etching apparatus, or a sputtering apparatus used in the manufacturing process of a semiconductor. In recent years, there has been a growing demand for a vacuum pump having an advanced function. For example, the development of a vacuum pump which provides a high ultimate vacuum is needed, because the process of manufacturing semiconductors has become highly integrated and fine-structured. In addition, vacuum chambers are becoming larger and larger as wafers and liquid crystal bases are becoming larger. Under these circumstances, it is necessary to use a large vacuum pump so as to increase a discharge speed of gas.
A screw thread groove type twin rotor vacuum pump which generates little vibration and noise is used in semiconductor-manufacturing process. As shown in FIG. 19, a conventional thread groove type twin rotor vacuum pump comprises two rotors 100a and 100b, accommodated in a casing 102, which rotate in opposite directions and have concave and convex grooves meshed with but not contacting each other. Gas is inhaled from an inlet 101 and discharged from an outlet (not numbered). The rotors 100a and 100b are fixed to each of shafts 103a and 103b. Ball bearings 105a and 106a support the shaft 103a. Ball bearings 105b and 106b support the shaft 103b. Timing gears 107a and 107b are disposed at the lower ends of the shafts 103a and 103b to allow the rotors 100a and 100b to rotate synchronously.
It is necessary to consider the capacity of a load to be applied to a bearing portion in designing the bearing portion for use in the twin rotor vacuum pump. The disadvantage of the conventional twin rotor vacuum pump is that a thrust load is applied to the bearing portion is greater than a radial load because the pressure difference between the upper surface and the lower surface of each rotor perpendicular to the shaft of each of the rotors 100a and 100b, namely, between a gas inlet side 111 and a gas outlet side 112, is applied to the rotors 100a and 100b as the thrust load. For example, supposing that the pressure difference between the gas inlet side 111 and the gas outlet side 112 is .DELTA.P=1 kg/cm.sup.2 and the diameter of each rotor is 10 cm, a thrust load of F=5.sup.2 * 3.14 * 1=78.5 kgf is applied to the bearing portion.
In order to eliminate this disadvantage, lubricating oil accommodated in an oil tank (not shown) is fed to the bearing portion. Thus, the lubricating performance of the pump is maintained under the condition in which a high load is applied to the bearing portion and the pump is continuously operated. But due to the increase in the number of rotations of the rotors or the manufacture of a large pump caused by recent demands for an advanced performance vacuum pump requiring increased gas-discharge performance made in recent years, more load is applied to the bearing portion. Under these circumstances, the development of techniques for securing the long life of the bearing portion is required.